The present invention relates to a method for operation of a synchronous compensator comprising a rotating electrical machine with rotor and stator with at least one winding with solid insulation enclosing the electrical field. The invention also relates to such a synchronous compensator with a measuring device to measure parameters relevant for the temperature conditions in the rotor.
Reactive power occurs almost always in electric power systems for alternating current transmission. Many loads consume not only active power but also reactive power, and to stabilize the voltage on the mains, the consumption of reactive power must be compensated for by production of reactive power. Under certain circumstances long high voltage transmission lines will also produce reactive power and require compensation to avoid raised transmission voltage.
To compensate such consumption or production of reactive power one can use static power compensating equipments (SVC) or synchronous compensators. The advantages of either of these techniques are discussed in e.g. the document WO 97/45922.
Synchronous machines designed with so-called cable technology, with at least one winding made of a flexible high voltage cable with solid insulation, have proven to be especially advantageous as synchronous compensators, see the above WO 97/45922 and WO 98/34312. Such machines can be designed for such high voltages, up to 800 kV, that they can be directly connected to any mains. This eliminates the cost of transformers as well as the reactive power consumption of the transformers themselves. A larger percentage of the reactive power produced can thus be fed to the mains. A synchronous compensator made with cable technology will also withstand higher overloads than conventional synchronous compensators, for short time and longer time overloads. This is primarily a result of the time constants for heating of the stator are longer due to the electric insulation of the stator winding. Further, while the power losses in conventional synchronous compensators are mainly losses in the conductors, these losses are smaller in synchronous compensators made with cable technology where the main part of the losses are core losses. Since core losses occur at ground potential they can be easier cooled away.
The part of the stator which is most critical from the temperature point of view seems to be the cable, and in the following the term stator temperature refers to the temperature of the cables.
The aim of the present invention is to provide a new technique utilizing said advantageous properties of machines of the actual kind to temporarily extend the power range for synchronous compensators with a rotating electric machine of this type.
The aim is fulfilled with a method and a synchronous compensator of the type described in the introductory portion and having the characteristics of claims 1 and 10, respectively.
With the method according to the invention, the cooling of the rotor is forced when the synchronous compensator is operating over-excited, depending on a value of the rotor temperature determined from measured parameters relevant for said rotor temperature. The forced cooling will be initiated when the rotor temperature tends to become too high, and in this way it is possible to utilize the advantageous thermal properties of the stator of a synchronous compensator made with cable technology, without the rotor of the machine restricting the possibility to utilize the overload capacity of the stator.
According to an advantageous embodiment of the method according to the invention, the machine is designed to permit overload also at under-excited operation. This allows a temporary extension of the range of operation also for under-excited operation. In this way, the range of operation for a synchronous compensator according to the invention can be temporarily extended from stationary +/xe2x88x92100% to e.g. +/xe2x88x92200% for a duration of the order of 30 minutes to 1 hour by forced cooling of the rotor and by dimensioning the machine to get 1/xq=2, where xq is the quadrature-axis synchronous reactance, i.e. in the direction of the pole aperture, in per unit based on the rating of the machine. Operators of power systems often specify that considerable overload capability may be needed for 15-20 minutes or more during operation disturbances, since that would allow the operator to take measures such as mains switches, start of gas turbines to ensure continuous operation with a minimum of disturbances for the customers.
According to another advantageous embodiment of the method according to the invention, parameters relevant for the temperature conditions in the stator are determined, and during over-excited and under-excited operation, the inductor current is reduced if a critical stator temperature is exceeded. In this way one can prevent the stator temperature (primarily the temperature of the winding) getting too high while operating the synchronous compensator in the extended range of operation.
According to still another advantageous embodiment of the method according to the invention, the machine is designed for a large short-circuit power. The result of such a construction is that the synchronous reactances become small, making it possible to extend the range of operation of the synchronous compensator for under-excited operation.
According to still another advantageous embodiment of the method according to the invention, the inductor current is reduced if the temperature value of the rotor exceeds a predetermined first limit value, and the cooling of the rotor is forced if the temperature value of the rotor exceeds a predetermined second limit value lower than the first limit value. Likewise, if the stator temperature exceeds a maximal permitted upper temperature limit of the stator, the inductor current is reduced.
According to another advantageous embodiment of the method according to the invention, the temperature value of the rotor is calculated from one or more of the parameters inductor current, inductor voltage, directly measured temperature on the rotor, temperature of the cooling medium and flow velocity of the cooling medium. There are thus several ways to determine the rotor temperature value which controls the forced cooling of the rotor. This value can be estimated calculated from the inductor current and inductor voltage. The temperature directly measured at a critical point on the rotor may be used to represent the rotor temperature. Alternatively both the calculated rotor temperature value and the actually measured value may be utilized as rotor temperature value, e.g. by forming a mean value, and the temperature and flow of the cooling medium can be utilized to get a more reliable rotor temperature value. Specific advantages are gained by using directly measured rotor (or stator) temperatures instead of values estimated from currents and voltages. Thus it is possible to take account of the initial temperature of the machine at the beginning of the overloading. If the machine has been idling and consequently is xe2x80x9ccoldxe2x80x9d when the overloading begins, it will be able to withstand more overload during longer time until maximum allowable temperature limits are reached. In this way one can reduce the conservative determining of limits for stator and rotor currents, which normally is applied in known technology.
To utilize the possibility of a considerably enlarged range of operation during under-excited operation, means are provided to permit negative inductor current. These means can comprise a static exciter with two bridges. By these means the range of under-excited operation may be extended to xe2x88x921/xq in per unit based on the rating of the machine and at nominal voltage without risk of exceeding the stability limit of the machine. xq in unitary values based on the rating of the machine refers to the crosswise synchronous rectance. According to an advantageous embodiment of a synchronous compensator according to the invention, a temperature estimator is provided to estimate a value of the stator temperature from one or more of the parameters stator current, stator voltage, directly measured sheet iron temperature of the stator, temperature of the stator cooling medium or flow velocity of the stator cooling medium. This makes it possible to monitor that the stator temperature, (primarily the temperature of the stator winding), does not become too high.
According to other advantageous embodiments of a synchronous compensator according to the invention, the forced cooling system is designed to force cooling of the rotor by lowering the temperature of the cooling medium, alternatively by increasing the flow of cooling medium. This offers two practical ways to realize the forced cooling.
According to still another advantageous embodiment of the synchronous compensator according to the invention the cooling medium is a liquid, preferably water. In liquid cooling the cooling medium can be directed towards the thermally critical points, permitting optimised cooling. Such directed cooling flow is not easily realized with gas or air cooling, why such cooling systems have to be oversized.
According to yet other advantageous embodiments of the synchronous compensator according to the invention, a wireless communication equipment is provided to transmit measured values from temperature gauges located on the rotor to the temperature estimator. The communication equipment is preferably arranged to transmit the measured values from temperature gauges on the rotor by so-called Bluetooth technique. Such a Bluetooth wireless transmission is insensitive to interference, and well suited for reliable transmission of measured data in highly disturbed environments, at the same time having such a short range that it is no source of disturbance for the surroundings.
According to still another advantageous embodiment of the synchronous compensator according to the invention, the winding comprises a flexible cable with an electrically conductive core surrounded by an insulating system of two semiconductive layers with intermediate solid insulation. In this way the electric field is substantially enclosed in the winding. The potential of the external semiconductive layer can then be substantially kept at ground potential, with resulting obvious advantages.